Cannot withdraw fluid (lava) with a bucket from a drum.
Tried left and right clicking. Sneak and not sneaking.

Most up to date version (Sky Factory 4.0.2)

i have had this issue also i initially assumed it was just intended when i was using wooden barrels but i was able to withdraw buckets of lava from an iron drum perfectly fine

that was until it filled up and it stopped letting me withdraw lava from it

also using the default version in SF 4.0.2

Can confirm:
1: You CAN withdraw from a iron drum filled with lava (the only fluid I've tested) IF it is not all the way full.

2: You CANNOT withdraw from an iron drum (full or not) if you have multiple buckets in your hand (a stack of 4 buckets)

3: You CAN withdraw from an iron drum (not full) if you have an item like a diamond bucket and you left/right click the drum.

Confirmed this happens in dev. I accidentally copied the insertion math as the withdraw cap. Should have a fix out later today.