No one could produce the sounds for my name in Thailand, so I just picked a nickname (as is the local custom) and I've rolled with that as my primary name ever since.
My favourite Thai pronunciation is the common Thai nickname “Apple” which is a loanword. However they pronounce it “ah-poon” because the character they use for the l sound (ล) is pronounced as an n in the terminal position. Also, Thais like to shorten words, so sometimes this nickname becomes simply Ple, pronounced “Poon” O_o
It's pretty obvious now though. Most all 'weird' Thai pronunciations for English come from transliterating English into the RTGS Romanization system (whose sounds don't align with English, like 'p' being ป, the unaspirated version of that sound which English doesn't have) an then translating from the Romanization to the Thai abugida to be pronounced. Speculating, since someone misunderstood stress syllables always put a falling tone on the final syllable and now you have the Thai pronunciation. Pepsi isn't translated as phonemes as เพพซิ, but p-e-p-s-i as ป-เ-ป-ซ-•ี; smash in that falling tone and you get เป๊ปซี่ which honest to Godzilla sounds like bep-SEE instead of PEP-si. Ask someone to read your ID and they'll use this same system to read it instead of trying to think about how English writing works.