STM32 Logic level high and low thresholds
The logic levels are described in section 6.3.14. When your supply voltage is 3.3V then:

V_IL = 0.39 * Vcc - 0.06 = 1.23 V (table row "I/O input low level voltage except BOOT0". The maximum voltage that will be read as logical zero is 1.23V (+ read all footnotes).

V_IH = 0.49 * Vcc + 0.26 = 1.88 V (table row "I/O input high level voltage except BOOT0"). You need to supply at lest 1.88 V to make the pin read as logical one.

Your 1.8 V signal is below V_IH and above V_IL, which means that the result is unpredictable.

You have to be concerned with the maximum voltage read as zero (ie. not exceed it) and the minimum voltage read as one, not the other way around.