I changed toner where everything was black

Ah, Brother; there's a quirk with their hardware that people don't always check until the thing throws an error or print issue. When you change the toner on the Brother printers, the corona wire has to be cleaned by using a slider on the imaging drum. Don't worry about missing it, every other split drum system (even combo toners) dropped this when PCRs became more common. Some kept it around for a while as a part of the sealed drum unit, but even they discontinued those parts at some point.
I've highlighted it, but you need to swipe that ~5 times. It doesn't always work, but it tends to clear up the issue on Brothers. The "cleaning blade" has to be returned to the original position or it has issues or will stop printing until you do it.

This is what every manufacturer does except Brother now:

This one came from my Lexmark laser; it was defective, so I pulled one from an MX AIO with shipping damage since it used the same part and gambled (sort of, I had one with PCR issues I didn't know about that were very visible and it took; shipped with a printer too) and it took. That black roller is the PCR, which everyone else (except Brother for some reason) has gone to. You will NOT SEE THIS corona design outside of Brother and maybe (and that's a stretch) some older copier designs that cannot be readily modified to use a PCR that the manufacturers still sell (or have to support for several more years because of contractual issues). The shift led to longer lasting drum units, but the corona wire masked print defects a little better. These PCR systems just let the defects show up very visibly so you either live with it if the drum is nearly due anyway or replace a drum with thousands of pages left. However when it's done with split drum units, at least you don't lose your toner over a print defect.

The other thing that can cause this is corona wire damage where it comes in contact with the drum such it short circuits. Unless you have corona wire and know how to take the drum apart to fix it, you have to replace the drum if the wire snapped.