Activities won't sync over bluetooth, only WiFi or USB.

It's a shame that after so many years of development, Garmin devices are still susceptible to intermittent FIT file corruption. The system software ought to have some sanity checking built in to prevent it from writing corrupt records in the first place. And Garmin ought to license the Fit File Repair Tool from Mathias Krallmann and integrate that technology into Garmin Connect to automatically repair corrupt files during sync operations instead of expecting customers to manually delete files. This is not a good user experience.

https://www.fitfilerepairtool.info/

In my experience, using the "Download HR" feature at the end of an activity to download stored heart rate data from an HRM chest strap seems to cause file corruption something around 10% of the time. It just happened to me again yesterday with an open water swim activity. I tried to contact Garmin Support about this a while back but because I couldn't reproduce the defect in any consistent way they just blew it off and didn't seem interested in doing a real root cause analysis.

If any watch is repeatedly having the issue, the watch will need to be exchanged. I am going to put the issue into a more realistic reality for yourself and anyone else that reads this thread.

There are literally millions of activities uploaded to Garmin Connect each day across both our Fitness and Outdoor segment. The issue of a watch developing a corrupt file is very, very rare. Any watch that has the issue ongoing should be exchanged.

I highly doubt that this is a hardware problem. My watch was already exchanged once for a different issue. As a software developer myself, all of the evidence I have seen points to some sort of intermittent software defect. Probably a memory allocation problem or something similar.

The issue of FIT file corruption is not as rare as you think. You are not seeing the extent of the problem because corrupt files don't sync to Garmin Connect in the first place, and most impacted customers don't bother to complain.

Why can't Garmin add some debugging code to automatically detect and report file corruption back to the developers (with user approval)? That would allow for investigating the root causes. And why can't Garmin automatically repair corrupt files when there is already a third party application which does so fairly well? I love Garmin products and want to see them succeed but the company has always had a disappointing lack of concern over software reliability and usability.

https://www.dcrainmaker.com/2019/06/competitor-software-instability.html

I am having exactly the same issue for the last 3 weeks. Went through the same steps as you did, unpairing, deleting from Connect, restarting watch, etc. While the connection seemed to be back to normal after re-pairing the watch, it did not last long. The strange thing is that I can see the heartrate live in the Connect app, confirming there is some data coming in from the watch through Bluetooth, just not the update from the activities I just finished. In my case, I have too changed the phone on which it was previously connected, going from an Samsung S20 FE to a S23 Ultra, the difference being that I had not noticed this issue on the S20, mind you, me changing my phone might have coincided with the update to 26.00 coming out.