Will I be liable for tax in Thailand if I re-enter on a 60-day visa exemption for medical treatment?

You should be more worried about if the Immigration officer will let you in on that 2. visa exempt.

If you get the 60 day visa exempt stamp and extend by a further 30 days at the Immigration office, Immigration class you as a tourist. If you then attempt to enter again on a visa exempt stamp, Immigration rightly consider that after on 90 day stay in the country that you are no longer a tourist but that you are attempting to live long term in the country. While there is no legal limit to the number of Visa exempt entries, once the Immigration officer believes that you are no longer visiting the country as a tourist he will deny you entry. If you have already stayed for 90 days on a visa exempt entry, then in future to guarantee entry into Thailand, you need to obtain a long term visa suitable to the reason for your stay.

No. Only liable for tax if staying for more than 180 days in a tax year.

Just because you may become tax resident in a tax year does not mean you will have a Thai tax liability.

@Huiying ****************

Highly doubt it one rule is you need to stay here 180 days. If unsure seek professional advice not here