We've deployed a preliminary patch that addresses the RBF transaction issue, and a full backend update with additional fixes will be released at the beginning of January.
Let's test this:
1. I created Offer ID: eb1bbd43-dace-44be-8d57-22343c7f185f (BTC > XMR), and deposited 0.001 BTC.
I bumped the fee from 1 to 5 to 10 to 20 sat/vbyte, before and after I clicked "Confirm deposit". Now we wait 
2. I also bought some BTC with XMR to have more test coins later

3. I also bought XMR with BTC by Accepting Offer ID: ae166f40-3f59-49f2-9ca8-88e96859c58f. I bumped the fee from 1 to 2 to 4 to 8 to 16 sat/vbyte. Now we wait

Results: For both BTC transactions, the last fee bump is the one that got confirmed.
1. This failed: "Deposit Timeout". But this was my fault, I messed up: the transaction was sent to my own address. I think it's because "Increase fee" and "Cancel (double-spend) are so close together in Electrum.2. My BTC arrived (txid: 4cd62bf1057340eaf0c9cab75cb5dcf87ad01bd8618560124e5e088832ffdbd5). This worked. BUT: 763 sats was still sent to another address. That's less than 0.1%, but for lower amounts (say $20 in Bitcoin) this would be significant if the same amount is turned to dust.
3. My XMR arrived.
Retry the first one:
4. I created Offer ID: b4365b63-9086-4658-a694-531dc3953ea1 (BTC > XMR), and deposited 0.001 BTC.
I bumped the fee from 1 to 5 to 10 to 20 sat/vbyte, then (and only then) I clicked "Confirm deposit". After this, I bumped the fee again to 50 sat/vbyte. Now we wait

Result: The transaction paying 50 sat/vbyte got confirmed and my Offer is Active. RBF seems to work okay now.