People get a UBI regardless of whether or not they work, that's the 'U' part. So any money you get from working is additional money on top of the UBI. It would be taxed, but it's still more money. It's not like other benefits where once you start earning your own money, you lose the benefit, and thus have to make a choice between not working to keep the benefit and working 40 hours a week to make similar or marginally more money.

Or, maybe you're taking that into account already, and are worried about an uncanny valley in between "not working" and "working" where a significant number of people simply don't work because that first rung in the "working" ladder isn't rewarding enough to start climbing the ladder, so they're incentivized to never work. If so, that's certainly a concern, but that would also be an indicator of a failed system that needed to be revised. It goes without saying that any implementation of a UBI system would have to be designed such that that didn't happen, but that's not an intrinsic quality of UBI systems.

Tax increases themselves would have to be progressive (as our current tax system supposedly is and always has been) to avoid creating these sorts of discontinuities that would disincentivize people from working. Any functional UBI system would have to take that into account, otherwise it's a failed implementation.