Is it only me that my hears are playing tricks? I have heard 'ยืม' pronounced as closer to 'ยีม' in real life.

You may be talking about different words

1) borrow ยืม

2) smile ยิ้ม

However - always ยิ้ม when your girlfriend wants to ยืม

It's always a bit tricky to discuss pronunciation in text. What may be throwing you off is that the vowel sound in 'ยืม' is an unrounded back vowel. Such sounds are the most common in East Asian languages, and not very common in the languages of Europe (there was a discussion before whether they exist at all but I can not remember - either way it is not very important as it's unlikely to help with the question you're asking).

Not so few foreigners mispronounce this vowel sound, often so it sounds more like an 'oo' than it should. Now, to get an 'oo' you will need some degree of lip rounding. So if you do round your lips, you are not saying the Thai sound correctly.

The Thai sound is produced by having your tongue at the same position as if you would be saying 'oo' - but your lips should be unrounded, even slightly spread - you can spread then to a thin smile and still pronounce it correctly - in fact, one of the exercises my teacher had me do when first working with this sound was to put a pen between my teeth, as that stops you from rounding your lips.

Spread lips is a feature of the front vowel 'ii' in many other languages (usually 'ee' (bee), 'ea' (sea) or 'e' (we) in English). So maybe your ear thinks it's hearing an 'ii' sound because of the lip spreading, and is simply not conditioned to notice that the speaker's tongue position is further back toward the throat than if she/he would be pronouncing an actual front vowel.