If I understand you correctly you issue is not about records disappearing randomly? Then this is not the right thread for your question.
Anyway: In your case it would probably be enough to change the update string your router sends to dynv6 on IP address renewal.
If you are using a Fritzbox then try and edit the update URL in the Fritzbox DynDNS settings. Replace <ip6addr> with <ip6lanprefix>. (Otherwise you might find an analogue setting for your specific router in its documentation).
Also make sure that your IPv6 record(s) in Dynv6 contain only the host part(s) of the respective IPv6 address(es), starting with “::” (two colons). The resulting IPv6 addresses are composed of the prefix and the host part. On the “Records” page, you can toggle the view by clicking on the double arrow icon in the right of “Data”.
Note that the records are not updated on renewal but only the prefix.
If this is not enough to help you please open a new thread as this issue is off-topic here.
Ah yes i am sorry i havent seen that in the update URL both parameters were implemented. So now it should work but without the ip6addr part the prefix isn’t renewed.
Sorry for that. I 'm very frustrated that I can’t find the bug.
I have the same problem with both an A and an AAAA record. They are updated daily and lost daily. It affects always the same records. Other records using the same ip seem to be stable.
Is this being investigated/worked on? It is quite annoying 
Same here. Regular losses of A and AAAA records.
Same happens with my only CNAME record. Daily lost and really annoying.
Edit:
Now it’s every hour.
Are there any news? This is a really serious and annoying bug which is present for ages…
I have the same issue.
Readded an AAAA record 2 days ago, set the Name to * and the Data the same as my IPv6 Prefix.
Today I noticed that it is gone again.
While I’m writing this it disappeared again!
Edit: Today “just” the “Name” has been purged.
I’ve had several records, including MX, disappearing from my domains recently. The same has happened to friends too. These issues have been going on far too long without being properly fixed. I know it’s a free service, but I think that the only viable solution is to move to another provider.
If anyone knows a service that supports IPv6 prefixes, please let me know
I lost the MX , 3 CAA , and 1 TXT record within the last 48 hours to add to the topic. I did have a zone with cloud in its name, deleted that in reaction to the latest posts but to no avail - pretty much annoying. 
Some records for several zones have also disappeared for me (AAAA for all zones(!) but not for subdomains, 1 TXT for zone). They have disappeared from the web frontend as well as from REST queries. But they resolve correctly on both google and cloudflare. Even the TXT record can be queried with nslookup/dig. Everything worked for years, the only new thing is the TXT record about 3 weeks ago. Never had subdomains with cloud in their name.
All very strange. All very annoying.
A couple of days ago my AAAA records were lost too. My two zones each contain only a AAAA record and a CNAME record. Admittedly a simple configuration, but a disappearing configuration is always annoying.
Because of this thread I began to develop a small REST client that checks the configuration of the zones and corrects them if needed. I use node.js to implement it. The first prototype has given positive results. I plan to use a cron job to call the client cyclically.
I think that I will publish a first version in GitHub in the next days or weeks. If you’re interested I can post the link to the repository once I’ve created it.
Me to , I lost many times
If someone wnats to go further:
Do you believe its working through a side effect? Someone maybe deletes unauthorized your entries? I made a deletion via ssh and a number of records were deleted which didnt belong to my account: Deleting not existing records via SSH
If someone wants eto explore deeper, i could help a little…
Have a nice day
I had the same problem here yesterday, but so far only this once.
It hit me too. For the second time, all AAAA entries of the ‘main’ subdomains were gone, the ‘sub’ subdomains were unaffected. As strange as annoying. Are there actually no statements from the operators?
The (only) TXT entry is gone too, again. Can it be due to TXT entries?
It happens all the time. This is just bs.
I need to recreate the records every single day because they will disappear every freaking day.
Same here.
nextcloud.mydomain.com and cloud.mydomain.com (domain edited) as AAAA entry get lost regularly and have to be recreated.
dynv6.com staff, please investigate this issue!
Same here. All record
Are they restored automatically?
Did i make a mistake?
A lost the records already for the second time.
This means al lot of trouble for me unfortunatlely and seems to happen not only one time.
Is there a more reliable solution here (or elsewhere)? Or did i made something wrong? Do i have to login and confirm the account after some time for example?