Filler

It's called flashing, it happens with certain fillers. You will need to spot prime the area in question with a good quality stain blocker like Zinsser Coverstain, followed by 2 or more full coats of paint. This can be purchased in spray form or in a tin from Screwfix, Toolstation and most DIY shops. I always carry a rattle can stain blocker. Unfortunately, your problem won't stop there, you may need to paint the whole area again (section of wall) as it won't touch in without showing up on previously newly painted wall.

Many top coats won’t cover filler patches which will lead to the filler flashing through the coat, touch up the patches with a contract matt or vinyl matt, then apply two coats of your chosen top coat which will ensure a solid finish.

Generally happens best options are to give good coat of primer followed by minimum 2 coats of emulsion,Matt will be fine after 2 but silk would need more.

Use a zinnserr stain block then spot in with colour your using then give it 2full coats this is called grining you can you use Toupet filler which does not grin

Use 20-180 grit sandpaper,make smooth,clear the dust afterwards,paint with oil base ZINSSER,just the patch.Apply twocoats of your wall paint over the primed pach.Let each coat dry properly

If you use filler, maker sure it’s smooth finish, if 2 coats of paint has not covered then use a stain block. Stain block will require 2 coats of paint of top to cover it.