You may see something like this in your log file, where the gap increases from 115% to 2292%.

Nodes | Current Node | Objective Bounds | Work

Expl Unexpl | Obj Depth IntInf | Incumbent BestBd Gap | It/Node Time

1503 1467 -283.00000 284 285 2160.99979 -285.00000 113% 609 127s H 1968 1870 1858.0000000 -285.00000 115% 662 145s [...] 4854 3038 -257.55772 357 368 13.00000 -285.00000 2292% 1044 919s H 4924 2950 8.0000000 -285.00000 3662% 1050 919s 5130 3120 -246.16119 393 407 8.00000 -285.00000 3662% 1044 942s H 5317 3172 7.0000000 -285.00000 4171% 1059 942s 5630 3377 infeasible 468 7.00000 -285.00000 4171% 1042 969s 5716 3420 -285.00000 21 472 7.00000 -285.00000 4171% 1058 993s 5937 3615 -284.00000 77 403 7.00000 -285.00000 4171% 1057 1024s H 5938 3615 2.0000000 -285.00000 - 1057 1024s [...] 8623 6061 -213.60040 141 748 2.00000 -285.00000 - 1219 1487s H 8848 6207 -103.0000000 -285.00000 177% 1210 1487s 9149 6446 infeasible 191 -103.00000 -285.00000 177% 1199 1561s 9675 6904 -280.00000 114 725 -103.00000 -285.00000 177% 1182 1626s [...] H18133 14145 -118.0000000 -285.00000 142% 1015 3309s 18258 14238 -264.98878 195 590 -118.00000 -285.00000 142% 1020 3408s 18590 14461 -198.90509 211 630 -118.00000 -285.00000 142% 1027 3622s H18749 815 -284.0000000 -285.00000 0.35% 1025 3622s

This is expected behavior in certain cases.

The relative MIP gap is computed as $$ \frac{|ObjBound - ObjVal|}{|ObjVal|} $$ where ObjBound and ObjVal are the MIP objective bound and incumbent solution objective, respectively.

There is one special case, namely if zero is a member of the interval between primal and dual bound. In this case, the interpretation of the MIP gap value may not make sense.

You can add a constant to your objective function that is large enough to make sure that the objective is always positive, but if you do this you will also modify the meaning of a gap. But if the objective function can have an arbitrary sign, then a relative gap is not that meaningful anyway.

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