I’ve tried to replicate the issue on my side but I couldn’t.
Could you please contact us through the help@bricksbuilder.io email, mentioning this forum post and sharing login credentials to your WP installation so we could investigate?
I have used the inline method to underline the text, which uses <u> to underline, and it saved successfully.
I later realized that I couldn’t even get the block editor to save my posts, so I had to install the classic editor to solve the issue. I figured that could be the reason for this bug. I have finished work on that site, though. Thanks