I’ve never had it happen with regular paint before but it does happen quite a bit with certain washes. Druchii violet springs to mind.

You could try putting a thin layer of matt varnish over the top to bring it back down. I keep a bottle of vallejo airbrush matt varnish handy for just an occasion, I just slap it on with a brush.

Many things can make it glossy or give it a satin finish.  Drying too quickly can do that, mixing paint systems by and large they play nicely together, but some don’t play well together i.e. Army painter and tamiya don’t play that nicely together as layer paints in my opinion (tamiya uses isopropyl alcohol as a thinner not water).  Different formulations some of the very bright mecha color range for example from vallejo can do this.  Contrast paints also seen to dry very glossy which makes sense as they work by staining and being translucent, half of them are closer to being inks not paints.

Did you wash the minis before priming and painting, release agent can do strange things to paint.  Soak in warm water with a bit of washing up liquid, or I use biological washing powder and a bucket and just soak in there for a few hours the rinse under the shower. You could use an ultrasonic cleaner too but the bucket is easier for me as I can just put the whole frame in and be done with it.

Do you have any agitators in your paints? M5 marine stainless steel nuts I use are good as they don’t clog the dropper of dropper bottles unlike glass beads or bb’s and are corrosion proof so won’t get eaten by the acrylic base.  I ask as some times the pigment will clump at the bottom and what your really painting with is acrylic medium with a hint of tint which dried glossy, conversely if the paint is old you may not have that much medium left in the suspension so it’s either time to replace, or top up with some medium, you can use lahmian medium but if your cheap like me just buy a half litre bottle of artists acrylic matte medium (liquid not gel) for under £10 and have a life time supply and add a drop or two with an agitator and shake like hell.

For fixing minis once dry your best bet is a matte varnish spray like dull cote, or a good matte acrylic varnish with an airbrush (I use vallejo) this can be a tricky thing to do as humidity greatly effects finish it can go cloudy which is worse than shiny in my opinion, don’t try and quick dry it either just give it a very light misting and leave alone for a few hours, repeat if still shiny, like most things you can easily add more not so easy to remove.

Finally as a last resort and you feel there is no way to salvage biostrip is a brilliant water based paint stripper no fumes very quick acting yet to find any issues using it on any material (although have heard of resin melting, maybe I’ve just been lucky) which will get your mini back to plastic/metal with a little scrubbing (use a child’s toothbrush won’t hurt any details) and washing. If you have one a water flosser or pressurised spray bottle (pump and spray) work great as mini pressure washers. Again a USonic cleaner can be used if you have one as a final clean before priming.

I was a massive advocate for Testor’s Dullcote when I first got my Quickshade Strong Tone “dip” – the brown polyurethane varnish from Army Painter, which dries *super glossy*. But now I’m not so sure. When I look back at my old models, they do indeed appear super-matt. But the colours are also a bit “dead” – they’re not so saturated as out of the bottle:

I painted a few up with edge highlights of the original base colours, and they really popped – but doing that so all my old minis felt like a massive time sink. So I popped a bit of satin varnish over the top. And, while they do have that satin sheen to them, the colours are much more saturated and interesting to look at.

In short, matt varnish makes colours look dull and lifeless – be careful with it!