In a heart that is working nominally, properly, a low heart rate is generally...not absolutely, but generally...nothing to be alarmed about. Many of us have low HR when resting, especially when asleep (but not when in REM sleep where our bodies can twitch, rapid eye movement, and we can moan or groan or utter unintelligible words and our heart rates can rise into the low 100's). If you are on a beta-blocker or calcium channel blocker, and your heart rate is under 50 much of the time, it might be an indication that your prescription needs a rethink, or at least a gander by the prescribing authority to confirm that the dose is correct. Things change, a heart becomes more efficient, gets better, less irritated, and then the current dose of BB can be too much. But, apart from keeping a record of daily/waking HR and BP, and looking for a trend toward lower numbers, I wouldn't worry for a second if my HR were at or above 40, and I felt good.

NB- Do be a bit more careful rising out of chairs and from the table, getting out of the vehicle, and arising from a prone/supine position getting out of bed. Take extra care to breath deeply two/three times, scootch your bum left and right, get some blood pressure up, and then slowly rise. Have a bail-out plan if you find yourself seeing spots and feeling lightheaded; be prepared to fall backwards, trying to keep your butt low to improve the distance you must fall if fall you must. Try also to keep your chin tucked toward your chest in case you fall back and slam a bit. Don't want a brain concussion as well as a sore butt/back.

Congratulations on a successful mitral valve repair. I think your heart is feeling improved and happier, and it needs to pump less forcefully. Assuming you aren't on any medication that might slow the heart or reduce its stroke volume, which beta-blockers will do, just keep an eye and if it gets worse, call a cardiologist right away.