Thank you for the post.
I created a sample file in FileMaker Pro 12 with the following requirements:
1. Layout with portal listing sessions (table 2) at an event (table 1).
2. Portal is filtered to show a subset of related records. (See screenshot below)
3. Two buttons on each row of the portal allow user to Reserve (+) or Cancel (-)
4. Logged in with the Admin Account
I am unable to reproduce the behavior described when logged into the Admin account when pressing either button.
Perhaps, I am missing a step to reproduce?
I recommend running a recover on this file to see if it might be damaged. Even if no problems are found by recovery, then I'd test the recovered copy for the same strange behavior.
Things to keep in mind about Recover:
While Recover almost always detects and fully corrects any problems with your file...
1. The recovered copy may behave differently even if recover reports "no problems found".
2. Recover does not detect all problems
3. Recover doesn't always fix all problems correctly
4. Best Practice is to never put a recovered copy back into regular use or development. Instead, replace the damaged file with an undamaged back-up copy if this is at all possible. You may have to save a clone of the back up copy and import all data from your recovered copy to get a working copy with the most up to date information possible.
And here's a knowledge base article that you may find useful: What to do when your file is corrupt (KB5421).
If recovery shows no errors, then can you create a new simple test file in FileMaker Pro 12 that replicates this particular behavior?