In my opinion, it would be a good idea to report the issue to NextCloud. However, this would have to be done by one of the Joplin experts. I would be willing to take part in the dialogue. However, I do not see myself in a position to represent this issue at NextCloud myself.

However, two aspects must be taken into account:

First:
Should the workaround suggested by roman_r_m, which could probably be implemented relatively easily in Joplin, be implemented relatively quickly at the same time?

Second:
Could it be that it is not the NextCloud people who have to solve the problem in their own code, because it could also be a 3rd-party library on the server side?