If you are still seeing piston movement, that would be the most likely place to start troubleshooting. If the belt is wedging a valve open you would lose compression.
I'm posting this as an answer, but it may NOT be the FINAL answer. This is just the first step in troubleshooting, so please hold off on accepting until we pinpoint the problem.
Thought you'd find something valve related....
Probably wear in the valve guides and/or stems if you are seeing it sticking. It's possible, however, that considering it's early in the season and I'm not sure how long it's been since it was last used the valve may have just been dry and stuck. The valve train get lubrication via oil vapors and can dry out over time when not running.
The only way to be sure is to do some disassembly and measuring. But the real-world answer unless it's for a critical job is to watch for any repeats and address it at that time if happens. Now you know exactly what to look for.
Hopefully, luck will be on your side. Best of success!