1. A one-sentence summary of bug: 
When you shoot enough mobs to level up the crossbow, it continues but goes into negative numbers.
2. Priority to fix? {High, Medium, Low}
Low-Medium
3. Which server? {Spawn, Redwood, Cypress, Maple, Willow, Expanse, DungeonRun, Rush, Creative}
Found on willow, could be a problem elsewhere as well
4. Steps to Reproduce: Please list the step-by-step methods to reproduce the issue
Get a Wildercrossbow, and get it to enough mobs shot to level up
5. Expected Outcome: What did you expect to happen?
Level increases
6. Actual Outcome: What happened instead?
Next level counter goes into negatives
7. Proposed Fix (Optional): Do you have a suggestion?

8. Notes (Optional): Anything else we should keep in mind in relation to this issue?
Crossbow is Reeseman5's, he has it and will need it fixed/replaced
9. Screenshots (Preferred): Do you have screenshots of the problem, proof or reference?

1. A one-sentence summary of bug: 

2. Priority to fix? {High, Medium, Low}

3. Which server? {Spawn, Redwood, Cypress, Maple, Willow, Expanse, DungeonRun, Rush, Creative}

4. Steps to Reproduce: Please list the step-by-step methods to reproduce the issue

5. Expected Outcome: What did you expect to happen?

6. Actual Outcome: What happened instead?

7. Proposed Fix (Optional): Do you have a suggestion?

8. Notes (Optional): Anything else we should keep in mind in relation to this issue?
Edit: Levels do actually increase, just the kills until next level and level counters are broken.
9. Screenshots (Preferred): Do you have screenshots of the problem, proof or reference?

https://imgur.com/gWAkkhC