Others have seen this looping issue with songs that have a Tempo change.
As a test, when looping, maybe try making sure there is a tempo marker right at the beginning of the loop.

Example
if looping bars 12 - 16 makes sure bar 12 has a tempo change entered for what tempo it was set to on the first pass.

It could be that a tempo change happens before bar 12,which is why it works first pass, but when it loops back the tempo never gets changed back from whatever it was at the end of bar 16
Hope that makes sense.

Solution would be for looping to look for the correct tempo setting on loopback instead of running linearly

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