Voltage drop across opposite diodes in series
(Update: we've fixed the convergence issue described below and the simulation now converges just fine!)
(CircuitLab simulator developer here.)
You appear to have discovered one of the drawbacks of Ideal Diodes. Their V-I curves are piecewise-linear and continuous, but not smooth. Not smooth means the derivatives are discontinuous.
This discontinuity can cause convergence issues, especially since you have two of these back-to-back. You'll see this noted with a yellow circle at the bottom right of the editing window, or with a message WARNING: nonlinear convergence failed. Be suspicious of results. at the bottom-right of the window if you explicitly click &quot;Run DC Solver&quot;.
I get this convergence failure warning when I try to simulate your original circuit:



simulate this circuit &ndash; Schematic created using CircuitLab
There are two possible solutions.
First, you can use P-N Junction Diodes, which are both continuous and smooth and will not experience this kind of convergence problem:



simulate this circuit
Alternatively, you can continue to use Ideal Diodes, but add some resistors that help the solver find a solution:



simulate this circuit
Either approach will eliminate this convergence issue, but using the smooth P-N Junction Diodes seems easier to me!