I should say these last loaves were made using Suas' formula for baguette with poolish. I also frequently use Reinhart's and Bertinet's formulas as well.
Any advice would be appreciated.
Not sure if the cause of your problems is the same as mine was but...
I, too, use the Reinhart and Bertinet formulas for French bread. As I started to make higher hydration breads (70% and higher), I was having a similar issue. The outside was browning nicely but the inside was damp and gummy, I assume from not fully cooking. If I increased the cooking time, the outside burned before the inside was fully cooked. Here are a few things I did that seemed to work for me:
1. Shape thinner loaves - I make 11-12" baguette because that's all my oven can fit. If I use more than 185g (6.5 oz.) of 70% hydration dough, I got wet, chewy centers. The more dough I used, the worse the problem became. Keeping dough weights at or below 185g and cooking 11-12" baguettes at 500F seems to yield the proper results.
2. Lower the cooking temperature - For larger loaves, I still pre-heat to 500 F but after a few minutes of cooking, I turn the temp down to 400 F. The lift is the same as cooking for 500F the entire time but overall cooking time gets increased due to the lower cooking temp. The inside temperature of the loaves gets higher at the end of cooking before the outside begins to burn and that eliminates the wet gumminess.
3. Reduce moisture in the dough - This is the least desirable fix, IMO, but it works. If I reduce the hydration to 66% or lower, I don't have any problems. For me, though, it rules out using the Bertinet slap and stretch technique. The drier dough is too stiff for me to effectively work that way. I go back to traditional hand kneading.
4. Let the loaves cool before cutting - Tempting, for sure, to cut the bread right out of the oven but if I let the loaves cool to room temp, I don't get the wetness inside. This is probably obvious but figured I would throw it in.
Your mileage may vary but I hope this helps in some way.
Nice looking bread! As you calibrate your process, it can help to take the internal temp of the bread to ensure it is fully baked. For rustic bread like that, it should be 200 - 210 degF. Loaf bread should be at 190 degF.
Here is an interesting article that helps explain the interaction of baking time and internal temp (in a fairly easy to understand way):
Agree it could be either not allowing the bread to cool before slicing or not baking long enough. However, based on the oblong shape of your cross section and somewhat compact crumb, you should also check that there is enough structure. It may be that conditions are more humid or your flour is a little variable and so the dough needs more kneading, an extra fold, or another minute of slapping :)
A less likely possibility - if you are following tried-and-true formulas - is that your poolish and dough have become too proteolytic and the proteins are degraded a bit too much. This could happen with overly warm temps or too long a ferment for the poolish, or a poolish that is too large in proportion to the main dough, etc.