เสียหน้า (to lose face) applies to both people in a conversation. Correcting you in public risks costing you face, and causing that is itself a loss of face for the corrector. The safest move for everyone is to praise your Thai and move on.
So you will be told your Thai is excellent from about your fourth word onwards, by people who did not understand you.
- เสียsǐːa
to lose, to spoil
- หน้าnâː
face
What to do about it
Do not take praise as data. Take being understood as data: did the person do the thing you asked, or hand you the thing you named? That is a signal praise can never give you.
And make correction cheap to give. Asking a specific question ('is it this tone or this one?') is far easier to answer honestly than 'is my Thai all right', because it does not require anyone to volunteer bad news.
This is the strongest argument for the field missions in this course: a transaction either worked or it did not, and no politeness can disguise which.