Automation item's LFO can become asymmetrical depending on some preference settings.
In the video clip above, there are two automation items' volume envelopes shown.
The first one is in fader scaling, and the second one is in amplitude scaling. BTW, you can change it from right click > envelope defaults > Volume envelope :.
Both are set as LFO type triangle and all other options are left alone.
Now if I go to change Volume envelope range from -inf..+6db to -inf..+12db, you can see the second volume envelope's LFO becomes asymmetrical.
So, this asymmetry seems to happen only when the envelope is in
1) amplitude scaling
2) volume envelope range : -inf..+12db
I had to investigate for a long while to figure out this issue.
Tested this on Reaper 6.13 and 6.14.