My 5C collets are all in excellent condition. Only if they've been used for short pieces quite often do they seem to get "sprung" open at the end. Even then, it's usually just worn to a bell-mouthed shape that causes parts to pop out. It's not a lot of grip anyhow with such a short length for the piece I want to make, so I plan to make very light cuts.

The ER collets are slitted alternately from top to nearly the bottom, and from the bottom to nearly to the top to give them that 1mm size range. Because of that range, they collapse easily upon tightening down the collet nut and if there's nothing to grip at one end you get a very cone shaped bore.

I've used a LOT of ER16 and ER20 collets to hold tools in CNC swiss-style lathes during my 30+ years work as a machinist, and never had any luck holding onto short tool shanks with them. My "rule of thumb" was that anything being clamped in an ER collet had to go in past at least half the bearing length of the collet bore. I never used them for holding onto work pieces, so things may be different (though I can't see how.)

This is a quote from the RegoFix catalog, and those are the highest quality ER collets and holders I've ever seen and used.

"Insert tool to the full length of the collet for best results
if possible. However, never insert the tool less than
2/3 of the collet bore length. Improper tool insertion
can permanently deform the collet and will result in
poor run-out."