Bulgaria to join euro area on 1 January 2026

This will likely come with a one-time significant increase in inflation, at least based on other European countries.

When Germany converted to the Euro, the conversion rate was (IIRC) about ~2 DM to the Euro but from what I recall, a lot of everyday things went from costing 7 DM to 7 euro, effectively doubling in price. IIRC France was similar (ie ~6.5 francs to the Euro but 10 Francs went to 3 euro, etc).

I've tried searching for any studies on this to see if the effect was measured and, if so, whether it held with later countries joining the euro.

I'm a little surprised that the euro has been this stable for this long (going on 30 years). Finland debated leaving. IT's debated if there's even a legal mechanism to leave. We still have the problem that the ECB sets eurozone monetary policy with Germany and Greece being vastly different economies.

Wasn't the lev pinned to euro for years, at the stable rate of 1 lev = 0.5 euro? Why the switching to euro would affect inflation much?

It's pinned at 1.95583 to the euro, not 2. If a merchant has a product that sells for 2 lev today, and they sell it for 1 euro tomorrow, they just increased the price by a bit over 2%.