French has no lexical stress. It has phrase stress, and it lands on the last full syllable of the group. You did not learn that rule and you cannot switch it off: it is what makes a French accent recognisable in every language a French speaker has ever spoken.
Put a Thai sentence in a French mouth and the last syllable gets lengthened and lifted. In French that costs nothing, because nothing in French depends on pitch. In Thai the last syllable is very often the politeness particle, and lengthening and raising it is exactly how you turn one particle into another.
- ค่ะkʰâ
female speaker, statement
falling. Add French phrase stress and it stops falling
- คะkʰá
female speaker, question
high. This is where the drift lands you
- ครับkʰráp
male speaker
high and short. French stress makes it long and emphatic
The second mechanism: speed
The words you know best are the ones you say fastest, and a tone contour needs time to happen. A rising tone that is not given room flattens into something a listener will hear as mid or as low.
So the drift is worst precisely on your oldest and most confident vocabulary, which is why it does not feel like a beginner's problem and why it survives into month six.
- ขาวkʰǎːw
white
rising, and the first casualty of speed
- ข่าวkʰàːw
news
low
- ข้าวkʰâːw
rice
falling. You say this one thirty times a day
Why you cannot hear it
You hear your own intention, not your own output. This is not a metaphor: the monitoring you do while speaking runs on the plan, not on the sound, which is why your recorded voice is always a small shock.
So record. A minute of yourself, played back the next day, will show you the drift in a way that no amount of careful attention while speaking ever will. Listen to the last syllable of each sentence first: that is where the French is.