Conditional formating for decimal places

Round wont help. try this

=--TEXT(A1,"#.0")

welcome to the forum, celphtitled. i think your decimal points are commas & your separators for formulas are semi-colons? just to let you know, most of us are using period & commas, so don't get confused. i don't really understand the question because of this:

This however does not apply if any other number after the comma has changed except the very first one, which is seen during the overview.

so if
A1:
33,4999084472656%
B1:
33,4499084472656%
it should NOT be highlighted? if so, try:
=ROUNDDOWN(A1,3)<>ROUNDDOWN(B1,3)

if it should be highlighted because A1 rounds to 33.5 & B1 33.4, then your formula is correct. except that this scenario is dealing with percentages. 33.5% is not 1 decimal point. it's actually 0.335. that's 3 decimal points
=ROUND(A1,3)<>ROUND(B1,3)

Welcome to the forum

Not sure if this is a typo or not?
=Round($A$1;1)<>ROUND($B1$1;1)

Should that be...
=Round($A$1;1)<>ROUND($B$1;1)