Since no reply so far for my problem, I made another test on Endurance Set CP160 jewellery.
Both rings and necklace had preliminary the below stats and I put all 3 to my character's gears before applying enchantments:


Basically I made a replacement of enchantments only on 3 consecutive jewellery parts
from their original effect "Adds 157 Health Recovery"
into new effect "Reduce Stamina cost of abilities by 191" (CP160 Purple glyph).

I made a check on DualWield abilities, and the below given cost was BEFORE enchanting jewellery:
Shrouded Daggers IV 2367
Bloodthirst IV 1596
Noxious Breath IV 1663
Steel Tornado IV 2235
Deadly Cloak IV 2235

After applying first enchantment with "reduction stamina cost by 191", only on Endurance necklace I got


And I got the following results on a.m. abilities:
Shrouded Daggers IV 2268 (-99)
Bloodthirst IV 1497 (-99)
Noxious Breath IV 1533 (-130)
Steel Tornado IV 2135 (-100)
Deadly Cloak IV 2135 (-100)
Screen of the Shrouded Daggers IV after 1-st enchant:


Now I enchanted 2-nd time - the first ring:


And got the following results:
Shrouded Daggers IV 2168 (-199)
Bloodthirst IV 1397 (-199)
Noxious Breath IV 1403 (-260)
Steel Tornado IV 2036 (-199)
Deadly Cloak IV 2036 (-199)
Screen of the Shrouded Daggers IV after 2-nd enchant:


Now I enchanted 3-rd time - the second ring:


And got the following results:
Shrouded Daggers IV 2069 (-298)
Bloodthirst IV 1298 (-298)
Noxious Breath IV 1273 (-390)
Steel Tornado IV 1936 (-299)
Deadly Cloak IV 1936 (-299)
Screen of all above abilities after 3-rd enchant:


This is a tricky part -> once I finished enchanting jewellery I logged off game and got even different results once I logged in again 1 minute later:
Shrouded Daggers IV 1995 (-372)
Bloodthirst IV 1224 (-372)
Noxious Breath IV 1176 (-487)
Steel Tornado IV 1862 (-373)
Deadly Cloak IV 1862 (-373)


To be precise I very carefully checked all my stats, buffs and any effects that could change the results between above steps - all stats were 100% the same.

Summarizing, 3 enchants with -191 stamina each should give -573 stamina on each ability, when my values differ much, even with difference between abilities.

As I read in Support answers here that "Players will notice, as they mouse over an enchanted item, that the tooltip indicates the effect of the enchantment."
and in another answer here that
"The following parts are considered major pieces, and receive 100% of the enchantment potency:
•Chest •Leg •Head •Jewelry •Weapons •Shields"


Moreover:
After I enchanted all jewellery, I also put new crafted CP160 Purple glyph (adds 902 max stamina) on my new crafted CP160 purple shield but I didn't notice any extra stamina added to my overall stamina pool (I had 30824 stamina before and after adding this glyph). It's like an enchant did not make any effect after it was applied to empty enchantment slot (shield had no enchantment yet).



I don't understand what is happening with this part of game and thus I ask anyone wise in this or better ZOS Support (maybe @ZOS_DaryaK) for help.
It seems if the enchanting system is damaged, the fun is also damaged since we create very expensive glyphs thinking they will do the effect as described, when in fact they cheat on stats.