Like you said, it's tough to figure out without the reasons

for changing the rule.

High school sports, like college and pros, should have a return-to-play policy for athletes returning from COVID, similar to how there are return-to-play policies for athletes returning from any injury. Such procedures might vary depending on whether the absence was due to close contact quarantine or positive isolation, severity of illness, duration of symptoms, etc. It might also vary on a sport-by-sport basis. The idea being to ease back into full-time play after a layoff.

That doesn't really help you here, especially without the reasoning behind the change, but I wouldn't assume one reason or another for a change in policy. Could be that the school wasn't following the state or local policy and just noticed, could be a change from above them, could be any number of reasons.