The code which decodes/encodes JOSE EC key outputs needs to be reviewed. The current code was meant to be a more optimal version of the code available in Jose4J (which is actually a slightly improved version of the code found in Santuario plus dealing with JOSE specific optimizations). This code needs to be double-checked and if needed reverted to a version closer to the one found in Jose4J.
Curves: it should be possible to deduce the values from the current EC key as opposed to having users configure them explicitly.
has just come out, so I guess EC code will need to be updated accordingly.
I also recall that if EC keys are loaded from JKS then the default algorithm needs be set correctly if EC signature/enc algo is not set, at the moment it is always set to RS256 by default