I know how to pronounce less. But in words with the suffix -less, it sometimes sounds like /lis/, other times like /les/. Which is true?
I think what you're observing is a change in progress. If you were to survey (UK) speakers a few decades ago, you'd probably find that most had /lɪs/. If you survey speakers today, you'll probably find most have /ləs/. (Except in the actual word "less", of course.)
A trend in English seems to be that, at least in the southern UK pronunciations, the language is moving towards having schwas instead of /ɪ/ in unstressed syllables generally.
The LPD-3 (Wells 2008) says the following:
-less ləs, lɪs. In singing, a strong-vowelled form les is usual.
Preference poll, BrE (for the word careless, disregarding votes for -les and from respondents who do not distinguish ɪ from ə in this position): -ləs 74%,-lɪs 26%."
Alan Cruttenden (2008) reports that /ləs/ is preferred by the younger generation in the UK.
cf. EPD-11 (Jones 1956) gives /lis/ only (e.g. for "careless") vs. EPD-15 /ləs/ as primary and /lɪs/ as secondary (Roach, Hartman, and Setter 2006).
The variation with this is very colloquial, so /les/ is more correct.
On a few minutes thought, so dont take it as gospel, it seems that the pronunciation is /les/ if preceded by a consonant e.g. regardless is more often /les/.
And the pronunciation is /lis/ if preceded by a vowel e.g. timeless is more often /lis/