Those reviews are from last year or more and recent firmware in May I not noticed any issues with toners but it's only June. That last review, I bet they had the print set to colour in the driver which will use the colour toners even if the document is mono only. I have one driver set as mono (default) and one as colour which makes toners last longer.
How long have you had yours? - it sounds like this "feature" was introduced in a relatively recent firmware update
Those reviews are not accurate.
The first two reviews on the Amazon page for this Brother printer from the same range suggest otherwise: https://www.amazon.co.uk/dp/B07HZPFBBP/?coliid=I3U97WWOOGH1LK&colid=37Q3RQC9I034C&ref_=lv_ov_lig_dp_it&th=1
Looks like printer prices gone through roof. I got brother LC8690 just before covid hit for £273 with £100 cashback. It's beast but very good printer