If you're looking at the option chain and the bid/ask changes, the IV for the bid/ask doesn't get immediately updated. It should, because it's a simple formula that can be executed on the client side.
Not always, the new distance of the strike to underlying is also a factor. It is also likely that the b/a changed based on parity values but the IV remained the same. Post an inside bid and see if it is reflected in the calculation. (FYI, sometimes IB will keep it internally than post to the market.)
I did that and it doesn't update the bid IV. The underlying can stay exactly flat and when new bids come in on the option chain it doesn't update IV automatically. I have to load a different tab of the option chain, and then come back to the original tab, to make it actually refresh the IV.