I doubt there is a problem.
The solar regulator
has no way of knowing the batteries are 100% charged. It has no communication with the battery monitor
Therefore it is trying to do an absorption cycle.
Typically this may to hold the batteries at 14.5v for a total of 1.5 hours (the handbook should tell you the exact time). Note it only counts the time at 14.5v if the voltage drops lower the clock stops.
The 80w solar panel will be able to maintain 14.5v when the fridge cycles off, but when the fridge compressor
cuts in the solar power is not enough to keep the batteries at 14.5v and the voltage drops.
In other words from the information you have given I think it is operating as it should do.
Given that you started with full batteries even though its operating correctly you are probably slightly overcharging your batteries. If they are wet cells I would not worry.
If they are AGM/gel you can usually force the regulator
onto float. The information should be in the handbook. If they are AGM
a bigger problem is that the absorption voltage may be too high, but its fine if they are flooded, which I presume is the case.