This is standard. While everyone is fighting the boss you just drink and get full mana. You have time between the pull and the first boss mechanic to drink and prep for the incoming damage. There are some exceptions but you’re just being butt hurt because they don’t do it the way you like.
The problem is you sat down OUTSIDE of the boss room to drink, always be in the room you can drink yet be ready to heal when needed. I’ve healed 20s where I’ve never had to drink, and 12s to where I’m OOM after every pull. Skill of the group you’re healing directly impact the amount of mana you have left at the end of the pull. 
A good tank watches the healer’s mana bar.
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I’m newer to healing this season, and am only in the 10-14 mythic plus level so far. As a mw monk I have found it somewhat easy to manage my mana without having to stop so much BUT if the group is not performing well then it definitely becomes an issue.
Like I can go a whole dungeon without stopping once with one group and then another I’ll find myself starving for mana before the first boss the next. It just depends!
Like I had a Paladin pull big in atal without many interruptions and standing in goo, ran into the totem boss with half mana and proceeded to not interrupt that boss as I helped a low dps on one of their totems.
Fun times. 
All of waycrests bosses, most of DotI, all of everbloom, last of brh, second and last of ToT, 1st and second of darkheart all have mechanics that can (in my case have) forced me to stand up 3~5 seconds into a pull, weither through aura, random mechanic targeting or pulse damage.
If this was the old days when water didnt scale id say those seconds should be able to cover fine, but with current water and especially if the group is bad enough like OP said to be running them that oom that often it probably wont cover any high damage bosses.
Also if theyre an established problem group always drink outside, itll save you the repair bill from the inevtiable wipe when you have no resources to heal.
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And this is why the Follower system will eventually kill M + as we know it today (Don’t kid yourself they will expand it)…in two years grouping is going to be rare to nonexistent…excuse me while I point and laugh.
Your toxicity pretty much killed Wow congratulations.
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Lacking on the reading skills I take it