how to reproduce:

create a world in 21w38a/21w37a, get a command block.

click it to open its GUI, then just type something in it such as "ablabla". Click "Done" button, notice that there's a message:"Command set: ablabla".

Then open the GUI again, "ablabla" is now gone.

if we put a command in it, it will behave normally until we open its GUI again.

Also, Needs Redstone/Always Active setting will also be forget. But the type of command block(Impulse/Chain/Repeat) will preserve (because they are different blocks), and the Conditional/Unconditional setting preserves too (because this is a block state, just like its facing). 

Based on these experiments, here's my interpretation: when player opens the GUI of a command block, the block-entity of the command block resets, but the block state preserves. According to MC-236663, if click cancel, the "change" does not replace the command inside before; if click done, the block-entity after reset saves and replaces the commands before.