This just happened to me. Finance analyst to senior analyst, title I've been chasing for two years. The raise they attached to it: 6%. For a title that, externally, would command 25-35% more.
I went back and asked about it directly. The framing from HR was "the promo brings you into the senior band, and your salary was already at the high end of the analyst band so the increase is smaller." Which sounds logical until you realize the senior band starts where my salary now sits, which is not how bands typically work.
Things I learned from asking around and some research:
This is common. Many companies treat promotions as primarily title events, not comp events. The comp catch-up is expected to happen over the next 1-2 performance cycles. Some companies even design it this way intentionally -- the promo signals you're on the trajectory, merit increases fill in the comp over time.
What you can do: Negotiate at the time of promo, not after. Once you accept, it's much harder. Ask explicitly: "What is the salary increase attached to this promotion?" before you sign or formally accept. If it's low, counter. The counter should be grounded in market rate for the new title, not your old salary.
If you're post-promo: request a compensation review at your six-month or one-year mark citing your new responsibilities and market data. External offers are still the most effective lever, unfortunately.
My offer: got the 6% with an agreement (in writing) for a compensation review at 6 months. Not perfect but better than nothing. We'll see.