I came across a sandhi rule - described here (point number 6):
A 2nd tone preceded by a 1st or 2nd tone and followed by another tone becomes a 1st tone
I never heard of this rule before so I was wondering if it is well known. I found one other reference for it here: Is there a tone sandhi for 2nd-tone-2nd-tone names?, where it's described differently:
In a trisyllabic expression, if (a) the first syllable is T1 or T2, (b) the middle syllable is T2, and (c) the final syllable is not weak, then the middle T2 can change to T1 in conversational speed.
Is this rule commonly known? Is it commonly accepted? Are there any other references for it?