This is almost for sure a body outgassing issue. Likely caused by the BISQUE firing from a lack of enough oxygen present for enough time, not from the glaze firing. The cumulative heatwork from the second glaze firing just allowed the more melted pyroplastic clay body to show what was hiding inside the body to start with.
You likely can't fix it at the glaze firing point.
Many "glaze firing defects" come from the bisque firing, not from the glaze firing. They only SHOW UP in the glaze firing.
Tightly stacked bisques, particularly fired in electric kilns, often have the issue of two things that combine to cause issues in the GLAZE firing. On is poor oxygen flow and dispersion in the load. The other is the thermal lag of the load. Put em' together.... and whammy.
There are numerous reactions that have to happen to the clay body in the bisque firing. Some require oxygen to be present (inside the clay body walls). Others just need to have enough time for evolving gases to migrate out thru the clay body. Some just need to have herat energy applied. All have a specific temperature or temperature range at which these reactions happen.
Cones are usually out in an "exposed area" (in the open) where you can easily see them. Thermocouple probes for controllers are also. These devices are measureing the heatwork and temperature in a location very different from the interior of the walls of a piece of ware, particularly if the loading of stacked up wares is densely packed. Therfore, sometimes the work is not fired to the heatwork that you THINK it is, no matter what you cones or controller are telling you. This issue is called "thermal lag". That's the amount the load is "lagging" behind the indicated temperature on a measuring device. There is ALWAYS some thermal lag.
Then there is the penetration of oxygen into the load. The edges of a stacking get this O2 pretty well. The interior of a dense load does not. If the kiln does not have good air circulation from an active draft of some sort, this issue gets worse. Electric kilns without local pickup vents are very difficult to get "right" with a dense load. Local pickup vents improperly installed or too small for the kiln unit are also causing issues with inadequate airflow.
If you tend to nest bowls one inside the other in large stackings in an attempt to have an efficient use of space... this can make this issue worse, affecting both oxygen penetration and thermal lag. It can also impede necessary outgassing.
This stuff is an IMPORTANT understanding to mastering firing operations. A main point from my ceramic materials and also kiln design and operation courses....... There is no such thing as a cookbook firing schedule. Firing is specific to the particular material being fired, the particular kiln being used, and the specific stacking job. You need to know when you have to alter thngs to get optimum results.
SOMETIMES, you can fix the poor bisque by dragging out the lower (bisque range) part of the GLAZE firing. Effectively you are doing what you should have done in the first place. Glaze firings are almost always stacked more openly....so heat penetration into the load and air circulation is better. However if you have a glaze application on the pieces that tends to become gas impermeable (particularly to O2) at a low temeperature (like American soda based shinos in highfire), then this can block the gas exchange.
Slow down your bisque firings, and make sure the local pickup downdraft vent is actually working as intended to get airflow. If this happens to you a lot, don't cram in as much ware in the bisque loads. If it is the occasional piece........ it likely was a happenstance of the way that one particular piece was located in the bisque firing....and comes under the category of "%$#@ happens." .
Hope that explanation helps.