You have also asked a similar question for the b1 meson and were told in the comments that the accuracy of measuring the masses of these resonances, and possibly the data available , is less than the minimum sigmas (of PDG fits to values of different experiments) away from each other. In this case the mass in the parenthesis is the mass of the resonance, and neutral to charged cannot be separated with any accuracy.
To get a feeling for magnitudes in MeV look at the $D^0$ mass and errors versus $D^+$ ,page 7 here for acceptable errors .
The mass difference is 4.822+/-0.015 MeV . It means that there are many experiments measuring masses with accuracy.
For the ones where there is no separation of the masses ,charged from neutral, in general it means that there is no statistical separation in the measured values, also possibly not enough experiments. The resonance signal is there, but the calculated errors are too large to separate the masses of charged to neutral with any accuracy.
The particular D you are asking about seems to have only a zero charged mode anyway, there is one experiment that has seen it and its decay mode is to a $D^*(2010)$ $π^-$. The width is large and with large errors, so it needs more experimental evidence, though it has been published. You have to go to the original paper to see whether it is an only neutral resonance or more measurements are needed to clear up its structure.