Good that you have found a solution.
It shouldn’t be needed but a few ISP’s do configure their dhcp servers in ways that are not in line with the various dhcp RFC’s. There have been a couple of other posts in the past where the ISP just refused to send any dhcp offer communication unless that rapid-commit option was disabled.
For the rapid-commit option, if the ISP’s dhcp server had that option disabled then it would just ignore the client rapid-commit request and follow the standard 4-message request approach. This is defined in RFC4039 which was released back in 2005.
You could try and follow this up with your ISP, it depends on how they respond to bugs flagged by customers.