Further thinking about this rule:
https://github.com/bitcoin/bips/blob/master/bip-0125.mediawiki4. The replacement transaction must also pay for its own bandwidth at or above the rate set by the node's minimum relay fee setting. For example, if the minimum relay fee is 1 satoshi/byte and the replacement transaction is 500 bytes total, then the replacement must pay a fee at least 500 satoshis higher than the sum of the originals.
The only time this error would ever occur, is when the original fee was <2 sat per byte.
Electrum appears to suggest increasing the fee by 1.5x, and this rule would always provide a high enough bump if the original fee were 2 or more sat/byte.
Example: 225 byte transaction @ 2sat/byte = 450 sat transaction
RBF Electrum - 1.5x fee = 675 sats (225 sats higher than the original fee, thus paying for its own bandwidth as per rule 4). <- Accepted
Example 225 byte transaction @ 1sat/byte = 225 sat transaction
RBF Electrum - 1.5x fee = 338 sats (113 sats higher than the original fee, thus
not paying its own bandwidth as per rule 4). <- Rejected
Example 225 byte transaction @ 10sat/byte = 2250 sat transaction
RBF Electrum - 1.5x fee = 3378 sats (1,126 sats higher than the original fee, thus paying its own bandwidth as per rule 4). <- Accepted
A simple fix would be to suggest the bump as: whichever is the higher of either [1.5x the original fee] or [an increase of sats equal to the bytes of the new transaction].