None of these uses are grammatical.
In the case of the first (which is from an answer on this site!) I believe the author meant to say
Meanings have been and are still in constant flux.
The addition of and makes sense: meanings have been constantly changing in the past, and they are still undergoing metamorphosis today.
The others, both from BBC News, appear to be the result of incorrect editing. It's possible that quotations can be "massaged" to fit a particular point in the news story, and the author couldn't decide which tense would fit best and then left both in the story; or decided to change what they had originally written and didn't take out the wrong word(s).
Quote 2 in context is
The police aren't stopping and searching people and I'm afraid to say that in the past the police have been are too keen to caution people. I think we need to make it clear — no more cautions for people found carrying knives.
This is obviously wrong, but the change of tense may have been unclear in the original speech and the sub-editor hasn't really helped to put it right.
Quote 3 in context is
"Real, Barca and Italian giants like AC Milan have achieved a level of revenue from TV deals that English clubs will only realise next season," said Fynn.
"That's because those overseas clubs have been are able to negotiate those domestic rights themselves and not collectively as the Premier League clubs do."
Again, I believe this is the result of a sub-editor's attempt to regularise the tense used in the quote.