Lumbar flexion in heavy lifts
Contrary to popular belief, some degree of lumbar flexion is not only safe in a heavy pull, it's actually necessary and is indicative of proper bracing. If you watch elite powerlifters squat and deadlift, you'll invariably see a bit of lumbar flexion. This pic is Lasha Talakhadze snatching his world record 220kg (485lbs) at last years IWF World Championship:
But everyone is different, and there's naturally going to be a bit of variance in the degree of flexion. Lasha here probably had to adapt to a bit more flexion than most because he's tall for a weightlifter (6'5").
This is why we don't teach "neutral spine." It's a useless cue. Neutral is a range, not a single position for all athletes in all lifts. Instead, we teach BRACING. That means taking a huge breath, pushing the stomach (not your diaphragm, but your belt line) outward as much as possible, then bearing down like someone's about you punch you hard in the stomach. When you do that, you'll naturally flex your lumbar spine a bit, but the intra abdominal pressure (IAP) you've just created will prevent excessive (dangerous) flexion. That IAP is integral for maximum force transfer from your feet through the bar.
Your shoulder blades (scapulae) should be depressed (pulled toward your butt), not retracted (pinched together). Retracting the scapulae elevates the rib cage and prevents optimal IAP. Depressing your scapulae contracts your lats very hard, which—in addition to improving IAP—assists in keeping the bar close to your body during heavy pulls. Notice Lasha is not arching his mid back. Again this is invariably the case among elite lifters in both weightlifting and powerlifting.
For your next PR, don't worry about a "neutral" spine. Worry about bracing correctly, as hard as you possibly can. That is how you'll safely move the most weight you're capable of.