Narrow shape (without color) on the paper. It's a mechanical problem

The first thing I always do is a "halfway test," which is when you pull the page before it hits the fuser and make sure the defect isn't imprinted in the fusing stage. If it doesn't occur, the pressure roller in the fuser is shot, or the fuser cover is doing it - this one is extra fun to isolate, given there are two parts that can cause this type of defect on the page, and one of them is tied to the fuser cover assembly. It's worse on these because Brother doesn't consider the fuser replaceable by the user(1), so you can't buy them directly from Brother - you need to take the back panel and a side panel off the printer, locate the P/N, and source it from an outside vendor. These machines are almost always a pain to a degree as well. Here is a link to the parts list to get the search going.

Brother Dcp L8400cdn L8450cdw Mfc L8600cdw L8650cdw L8850cdw L9550cdw Service Manual

Brother Dcp L8400cdn L8450cdw Mfc L8600cdw 8l8650cdw L8850cdw L9550cdw Parts Manual

(1): My Lexmark MS621 and MS631 I scored due to a bad touchscreen for scrap pricing I bought on impulse for $59 when the unit is worth $400+ WORKING(!!!), also have "non-user serviceable" fusers - but that's BS as far as I'm concerned. If you line the gear up and put the printer on an even surface, I can do the fuser in 10 minutes or less. Resetting it is as easy as going into the maintenance menu, which can EASILY be done from the main boot printer menu, or through the "service mode" methods. It's because Lexmark has it such that the wiring is error prone to the point the thermister is separate from the big wire and missing it hard stops the printer. It's effectively a skillset deterrent, given that Lexmark has a parts site where I can at least get the kit P/N if I can't buy the fuser. The monochrome "Go line" printers are the same as the MS - 10 minutes. Color "Go line" machines require top panel, side panel, and back removal, so it's a bit less accessible. It's due to the gear and thermister on monochrome machines (B/MS/MX); that's legitimately it.

Lexmark lets us do rollers without them, but not fusers. The 7, 8 and 9 series have user swappable fusers which do 300k+ pages - the 11x17 and 12x18 units are good for 350k pages before they need fusers. The big CS/CX units like the CS5/6 and up have MS/MX tier fuser serviceabliliy short of removing one side panel.

If the fault occurs at the ITB/drum point, I would look at the meter and roller condition - worn rollers can sometimes do this, even bad separator pads. Check for wear or damage to the supporting parts, such as the metal bar and chassis of the printer. If you can find them cheap enough and hopefully get genuine Brother rollers, I'd try and replace them as a blind luck attempt and see - it may be enough. If it isn't enough, there's a more serious mechanism damage issue somewhere needing heavy disassembly.