It has always been like that. When using the relist function, you are asking eBay to relist exactly as it was first listed - and that includes the original quantity.
That is why most sellers selling multiple quantities of the same item in a listing use GTC (Good Till Cancelled) duration instead of 30 days.
GTC costs the same - listing fees are charged every 30 days when the listing is automatically relisted - but only relists the quantity still available at the end of the 30 days.
So if you start with a quantity of 5 units and sell 2 units the first month, GTC will automatically relists a quantity of 3 units, etc... until all sold or revised or cancelled.
One problem of course is that when eBay offers "free listing" promotions for Fixed Price listings, GTC is not available. Durations are usually limited to 3,5,7,10 and 30 days on promotions with GTC specifically excluded.
Why is GTC excluded on those promotions?
Because in the early days too many sellers assumed, despite clear disclaimer to the contrary, that GTC meant free insertion fee forever (as long as the listing was available).
eBay solved that reading comprehension problem by speficically excluding GTC from promotions.
"I don't like how the quanity resets when relisting"
If you use the "relist" function, the quantity is automatically entered as it was when you first listed the listing.
For example, if you first listed 5 items available, eBay will show 5 items available if you use the "relist" function and will not make adjustments for any changes that may have taken place in the meantime (sales or additions/substractions to inventory).
On the other hand, if you list with GTC duration, the system automatially relists whatever quantity was available at the end of the 30 day period.
