Errors with Training Load on 4-Week Timeline Graph

I see some suspicious zero-miles rides in the stats. Indoor rides? Did you use some 3rd party app to record them?

The November 12th and 13th ride in that screenshot were indoor, yes. But no to the third party app. I still used my Edge 530 to record them. 

I eventually went back to manually added the mileage after the fact, but this never had an effect on training load associated with the ride as far as I'm able to tell. The spikes were present immediately upon sync.

Did you record the indoor rides using the Indoor Cycling activity, or just the standard outdoor cycling profile? I recommend using the dedicated Indoor Cycling profile. Also consider mounting a speedsensor on the flywheel, or even on the pedals, and calibrate it so that it reports around the same distance as the bike.

I did use the indoor profile, so the GPS was turned off on those rides to avoid any junk data/battery drain.

I unfortunately do not own the cycle I'm using at the moment because I'm away on a business trip and it belongs to the gym. So I won't be able to modify the machine. It does have cadence and power output, but no speed sensor that my Edge can detect.

If I count well, the peeks are from 12th and from 14th of November, while the stats show no activity recorded on 14th, which is strange. It looks to me as if the file with the metrics synced to GC was corrupted. Does the graph look the same when you look at it in the Reports section of Garmin Connect Web?

You could try saving, deleting, and then reimporting the activities from the concerned days (and perhaps also the one from the 13th) - export them from GC Web as "Original", then delete them, sync couple of times, and then see whether anything changed in the Training Load stats. Then import the activities back (after unzipping them).

The spikes occur in the 13th and 15th.

The 13th has an activity logged, but you can see in the Exercise Load graph, it was assigned a load value of only 67.

The 15th I logged no activity, and the spike occurs again.

Even if the spike were due to data corruption, the "snap" back to the original value on the 14th should not occur, because that graph shows a 7 day moving sum of training loads.

I'm not keen on deleting and re-uploading the rides, because Garmin has another known issue of retaining training loads for deleted activities, that I'm not sure is resolved yet.

Did you check it in Garmin Connect Web? Are the peaks there too?

https://connect.garmin.com/modern/report/-1/all/last_four_weeks 

Check also the cycling-specific report, whether it differs:

https://connect.garmin.com/modern/report/-1/cycling/last_four_weeks 

It can be still a data corruption. The acute load is actually not calculated by the app, or on Garmin's servers, but on the device. You should be able to see it on it too. I am not familiar with your Edge model, but on the watches, the Training Load graph is on one of the screens of the Training Status widget. If it is wrong there too, then it looks like the file holding this metrics got somehow corrupted.

The Edge 530 does not have a graph of previous loads available like the Connect applications do unfortunately.

There's a place where one is supposed to be, but it never has any bars displayed on it. I assume this is another software issue that Garmin is not interested in solving.

On sync, the Acute load updated to 223 on the Edge as shown in the picture on my first post.