During rainstorms, If a item gets thrown in lava, it bounces off of the lava, which gives the rain enough time to extinguish it before it burns.

Update (6/??): One way to fix is to prevent items from bouncing on the lava so that the lava can burn the items during rainstorms (more like a parity change from java)

Update No. 2 (9/28): I did this in 1.16.40 and now all non-netherite items burn from the lava in the rain as expected (due to a change where items no longer bounce off of the lava), so this should be marked as "fixed" from 1.16.20 if possible.