Base runs now being recorded as tempo/high aerobic

Trainning load isn't zone related, it's based on HRMax (<80% i think)

Check if your zones didn't get screwed up when your max HR changed (happened to me recently) - it shouldn't happen since they are based on LT but...

They also changed the auto Heart Rate detection in v23.00. Check it you Max HR is properly set because it might have been lowered automatically during one of you runs since v.23. 

Compare this run:

https://i.imgur.com/a21Kxvp.png

https://i.imgur.com/eMnbz6m.png

Improving base, zones more or less identical, higher average heart rate if anything, to this today:

https://i.imgur.com/koCSwyG.png

https://i.imgur.com/QfiCbvd.png

More time in zone two. I've been doing runs like this for nearly a year specifically for low aerobic benefit, and now it's totally changed.

See my reply above, my zones haven't changed and my max is based on what the strap detected. I'm using % lactate threshold anyway, so max heart rate should be irrelevant?

Zones are irrelevant in Training Effect calculations they are for visual representation for you.
Firstbeat algorithm that determines the Training Effect is using the percentage of Max heart rate. For Example for Base run your Average heart rate of the run should be below 80% of Max heart rate if i'm not mistaken. 

I spoke about latest update because(v23.00) if you have auto max heart rate detection enabled in v23 it is possible that watch altered the Max HR. And that would explain why now you have Tempo instead of Base.
In my case i use chet strap and had max HR of 200 this year and this new auto-detection algorithm changes my max HR very ofter to values between 197-199. 

Right, but my max heart rate hasn't changed in 6 months.

Look at the two runs above, both max heart rate identical, zones are 1bpm different (to account for new lactate threshold readings), yet they give different training effects.

Garmin gives me a max heart rate of 189bpm, so 80% is 151bpm. The run that gave me base had an average HR of 150bpm, and the one that gave me tempo is an average of 148bpm.

there's no Max HR value in your screenshots.
I've been seeing fews similar threads in the past that's why i thought your max HR got changed in the user settings. A part from that I have no other tips for you I'm sorry.  

I have exactly the same issue following upgrade to v23. I too use HRM Pro, and zones based on lactate threshold - and all the runs (HR/pace) that I previously did as base training are now registering as tempo. So weird. Garmin - what’s up with this?

Exactly same situation here...something has changed and it's wrong

I have similar problems now and then. On one occasion my HRM Run had a bad battery and after changing the battery things improved. But I still got the problem now and then.

In version 23.01 Alpha there might be a fix for this. Hopefully. Because the problem screws up the overall Training Status.

From 23.01 Alpha release notes:

"Fixed issue that could cause inaccuracies in the reported primary benefit for Training Effect"