I have observed some English speakers in North America who seem to produce this assimilation in words like "running" /ˈrʌnɪŋ/ (as /ˈrʌnin/) or "winning" /ˈwɪnɪŋ/ (as /ˈwɪnin/). I'm specifically interested in when the final vowel becomes /i/ or /iː/ not /ɪ/ (as in some Southern American English accents which produce "runnin'" and "winnin'").
Is this a regional accent?
I found some additional anecdotal discussion at the following links: